Non-impact printers such as xerographic printers are coming into more frequent use as a result of the advances in technology relative to printers. One such type of printer is a xerographic printer in which light emitting diodes (LEDs) are enabled in a selected manner to create an image on a photo conductive surface on a rotating drum or belt to produce a plurality of dots thereon which collectively combine to form the characters of a printed text or other graphic representations. In such a device, an input is supplied to the electronics of the print head which controls the enabling of the LED's, or the like, so that the text being inputted will be reproduced in a charged xerographic surface in the form of the image. This image may then be developed and transferred onto paper, or the like, as is well known in the art.
One of the problems with prior LED or other dot matrix type of non-impact printers is that a large array of LED's or elements must be positioned in a line across the surface of a rotating drum or a moving belt in order to place an appropriate number of dots to form letter images having high resolution. It would, for instance, be desirable to have letters which approach preformed impact letters such as found in many printers and typewriters, so as to avoid being readily identifiable dot matrix type printing. To improve the resolution requires that a larger number of LED's be accurately positioned adjacent each other in a small space. Physical limitations in current technology limit the number of LED's that can economically be placed in a small space and this will not reproduce an image of the high quality desired. In addition, if one or more LED's becomes disabled, the image is degraded and generally the whole LED array must be replaced. To overcome the limitations on space in setting forth an array of LED's it has been suggested to align the LED's in a plurality of rows, each row having the LED's offset from the previous LED's. It has also been suggested in the past to use various types of polymer lens for forming the focal points and otherwise improve the characters being reproduced by the printer.
In xerographic type printers, a photo receptor drum or belt has a photo receptor coating on its surface, which can be charged. The charged surface then has dots or other images placed on the surface by discharging the image area by an array of LED's or a rotating laser or other device. The image placed on the drum or belt is then developed with a toner, such as a powdered graphite toner, which is then applied to paper having moving contact with the rotating drum or belt. The image is then transferred to the paper using a transfer corona, or the like, and the image on the paper is fused thereto. The drum or belt has its photo receptor coatings cleaned to remove the image and charge from the surface thereof prior to it being recharged. This type of image transfer is commonly used in xerographic copy machines, except that rather than having a rotating laser or an LED array, the copying machine includes a direct transfer of an image through a lens onto the photo receptor surface. There are, of course, other well known means for electrically or magnetically transferring images to a drum or belt for transfer to a sheet of paper.
The present invention is aimed at a non-impact dot matrix type printer with an improved image for the number of dot producing elements being utilized by having an array of dot producing images shifted or a rotating drum shifted during successive rotations to utilize the same dot producing array in a different position to continue placing the image on the drum or on a moving belt. This inherently slows the speed of the printing but allows an improvement in the printed image and the production of high quality non-impact printer with high reliability and fewer components.